feature_test_macros(7) Miscellaneous Information Manual feature_test_macros(7)
NAME
feature_test_macros - feature test macros
DESCRIPTION
Feature test macros allow the programmer to control the definitions
that are exposed by system header files when a program is compiled.
NOTE: In order to be effective, a feature test macro must be defined
before including any header files. This can be done either in the com-
pilation command (cc -DMACRO=value) or by defining the macro within the
source code before including any headers. The requirement that the
macro must be defined before including any header file exists because
header files may freely include one another. Thus, for example, in the
following lines, defining the _GNU_SOURCE macro may have no effect be-
cause the header <abc.h> itself includes <xyz.h> (POSIX explicitly al-
lows this):
#include <abc.h>
#define _GNU_SOURCE
#include <xyz.h>
Some feature test macros are useful for creating portable applications,
by preventing nonstandard definitions from being exposed. Other macros
can be used to expose nonstandard definitions that are not exposed by
default.
The precise effects of each of the feature test macros described below
can be ascertained by inspecting the <features.h> header file. Note:
applications do not need to directly include <features.h>; indeed, do-
ing so is actively discouraged. See NOTES.
Specification of feature test macro requirements in manual pages
When a function requires that a feature test macro is defined, the man-
ual page SYNOPSIS typically includes a note of the following form (this
example from the acct(2) manual page):
#include <unistd.h>
int acct(const char *filename);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
acct(): _BSD_SOURCE || (_XOPEN_SOURCE && _XOPEN_SOURCE < 500)
The || means that in order to obtain the declaration of acct(2) from
<unistd.h>, either of the following macro definitions must be made be-
fore including any header files:
#define _BSD_SOURCE
#define _XOPEN_SOURCE /* or any value < 500 */
Alternatively, equivalent definitions can be included in the compila-
tion command:
cc -D_BSD_SOURCE
cc -D_XOPEN_SOURCE # Or any value < 500
Note that, as described below, some feature test macros are defined by
default, so that it may not always be necessary to explicitly specify
the feature test macro(s) shown in the SYNOPSIS.
In a few cases, manual pages use a shorthand for expressing the feature
test macro requirements (this example from readahead(2)):
#define _GNU_SOURCE
#define _FILE_OFFSET_BITS 64
#include <fcntl.h>
ssize_t readahead(int fd, off_t *offset, size_t count);
This format is employed when the feature test macros ensure that the
proper function declarations are visible, and the macros are not de-
fined by default.
Feature test macros understood by glibc
The paragraphs below explain how feature test macros are handled in
glibc 2.x, x > 0.
First, though, a summary of a few details for the impatient:
• The macros that you most likely need to use in modern source code
are _POSIX_C_SOURCE (for definitions from various versions of
POSIX.1), _XOPEN_SOURCE (for definitions from various versions of
SUS), _GNU_SOURCE (for GNU and/or Linux specific stuff), and _DE-
FAULT_SOURCE (to get definitions that would normally be provided by
default).
• Certain macros are defined with default values. Thus, although one
or more macros may be indicated as being required in the SYNOPSIS of
a man page, it may not be necessary to define them explicitly. Full
details of the defaults are given later in this man page.
• Defining _XOPEN_SOURCE with a value of 600 or greater produces the
same effects as defining _POSIX_C_SOURCE with a value of 200112L or
greater. Where one sees
_POSIX_C_SOURCE >= 200112L
in the feature test macro requirements in the SYNOPSIS of a man
page, it is implicit that the following has the same effect:
_XOPEN_SOURCE >= 600
• Defining _XOPEN_SOURCE with a value of 700 or greater produces the
same effects as defining _POSIX_C_SOURCE with a value of 200809L or
greater. Where one sees
_POSIX_C_SOURCE >= 200809L
in the feature test macro requirements in the SYNOPSIS of a man
page, it is implicit that the following has the same effect:
_XOPEN_SOURCE >= 700
glibc understands the following feature test macros:
__STRICT_ANSI__
ISO Standard C. This macro is implicitly defined by gcc(1) when
invoked with, for example, the -std=c99 or -ansi flag.
_POSIX_C_SOURCE
Defining this macro causes header files to expose definitions as
follows:
• The value 1 exposes definitions conforming to POSIX.1-1990
and ISO C (1990).
• The value 2 or greater additionally exposes definitions for
POSIX.2-1992.
• The value 199309L or greater additionally exposes definitions
for POSIX.1b (real-time extensions).
• The value 199506L or greater additionally exposes definitions
for POSIX.1c (threads).
• (Since glibc 2.3.3) The value 200112L or greater additionally
exposes definitions corresponding to the POSIX.1-2001 base
specification (excluding the XSI extension). This value also
causes C95 (since glibc 2.12) and C99 (since glibc 2.10) fea-
tures to be exposed (in other words, the equivalent of defin-
ing _ISOC99_SOURCE).
• (Since glibc 2.10) The value 200809L or greater additionally
exposes definitions corresponding to the POSIX.1-2008 base
specification (excluding the XSI extension).
_POSIX_SOURCE
Defining this obsolete macro with any value is equivalent to
defining _POSIX_C_SOURCE with the value 1.
Since this macro is obsolete, its usage is generally not docu-
mented when discussing feature test macro requirements in the
man pages.
_XOPEN_SOURCE
Defining this macro causes header files to expose definitions as
follows:
• Defining with any value exposes definitions conforming to
POSIX.1, POSIX.2, and XPG4.
• The value 500 or greater additionally exposes definitions for
SUSv2 (UNIX 98).
• (Since glibc 2.2) The value 600 or greater additionally ex-
poses definitions for SUSv3 (UNIX 03; i.e., the POSIX.1-2001
base specification plus the XSI extension) and C99 defini-
tions.
• (Since glibc 2.10) The value 700 or greater additionally ex-
poses definitions for SUSv4 (i.e., the POSIX.1-2008 base
specification plus the XSI extension).
If __STRICT_ANSI__ is not defined, or _XOPEN_SOURCE is defined
with a value greater than or equal to 500 and neither
_POSIX_SOURCE nor _POSIX_C_SOURCE is explicitly defined, then
the following macros are implicitly defined:
• _POSIX_SOURCE is defined with the value 1.
• _POSIX_C_SOURCE is defined, according to the value of
_XOPEN_SOURCE:
_XOPEN_SOURCE < 500
_POSIX_C_SOURCE is defined with the value 2.
500 <= _XOPEN_SOURCE < 600
_POSIX_C_SOURCE is defined with the value 199506L.
600 <= _XOPEN_SOURCE < 700
_POSIX_C_SOURCE is defined with the value 200112L.
700 <= _XOPEN_SOURCE (since glibc 2.10)
_POSIX_C_SOURCE is defined with the value 200809L.
In addition, defining _XOPEN_SOURCE with a value of 500 or
greater produces the same effects as defining _XOPEN_SOURCE_EX-
TENDED.
_XOPEN_SOURCE_EXTENDED
If this macro is defined, and _XOPEN_SOURCE is defined, then ex-
pose definitions corresponding to the XPG4v2 (SUSv1) UNIX exten-
sions (UNIX 95). Defining _XOPEN_SOURCE with a value of 500 or
more also produces the same effect as defining _XOPEN_SOURCE_EX-
TENDED. Use of _XOPEN_SOURCE_EXTENDED in new source code should
be avoided.
Since defining _XOPEN_SOURCE with a value of 500 or more has the
same effect as defining _XOPEN_SOURCE_EXTENDED, the latter (ob-
solete) feature test macro is generally not described in the
SYNOPSIS in man pages.
_ISOC99_SOURCE (since glibc 2.1.3)
Exposes declarations consistent with the ISO C99 standard.
Earlier glibc 2.1.x versions recognized an equivalent macro
named _ISOC9X_SOURCE (because the C99 standard had not then been
finalized). Although the use of this macro is obsolete, glibc
continues to recognize it for backward compatibility.
Defining _ISOC99_SOURCE also exposes ISO C (1990) Amendment 1
("C95") definitions. (The primary change in C95 was support for
international character sets.)
Invoking the C compiler with the option -std=c99 produces the
same effects as defining this macro.
_ISOC11_SOURCE (since glibc 2.16)
Exposes declarations consistent with the ISO C11 standard.
Defining this macro also enables C99 and C95 features (like
_ISOC99_SOURCE).
Invoking the C compiler with the option -std=c11 produces the
same effects as defining this macro.
_LARGEFILE64_SOURCE
Expose definitions for the alternative API specified by the LFS
(Large File Summit) as a "transitional extension" to the Single
UNIX Specification. (See ]8;;http://opengroup.org/platform/lfs.html\http://opengroup.org/platform
/lfs.html]8;;\.) The alternative API consists of a set of new ob-
jects (i.e., functions and types) whose names are suffixed with
"64" (e.g., off64_t versus off_t, lseek64() versus lseek(),
etc.). New programs should not employ this macro; instead
_FILE_OFFSET_BITS=64 should be employed.
_LARGEFILE_SOURCE
This macro was historically used to expose certain functions
(specifically fseeko(3) and ftello(3)) that address limitations
of earlier APIs (fseek(3) and ftell(3)) that use long for file
offsets. This macro is implicitly defined if _XOPEN_SOURCE is
defined with a value greater than or equal to 500. New programs
should not employ this macro; defining _XOPEN_SOURCE as just de-
scribed or defining _FILE_OFFSET_BITS with the value 64 is the
preferred mechanism to achieve the same result.
_FILE_OFFSET_BITS
Defining this macro with the value 64 automatically converts
references to 32-bit functions and data types related to file
I/O and filesystem operations into references to their 64-bit
counterparts. This is useful for performing I/O on large files
(> 2 Gigabytes) on 32-bit systems. It is also useful when call-
ing functions like copy_file_range(2) that were added more re-
cently and that come only in 64-bit flavors. (Defining this
macro permits correctly written programs to use large files with
only a recompilation being required.)
64-bit systems naturally permit file sizes greater than 2 Giga-
bytes, and on those systems this macro has no effect.
_TIME_BITS
Defining this macro with the value 64 changes the width of
time_t(3type) to 64-bit which allows handling of timestamps be-
yond 2038. It is closely related to _FILE_OFFSET_BITS and de-
pending on implementation, may require it set. This macro is
available as of glibc 2.34.
_BSD_SOURCE (deprecated since glibc 2.20)
Defining this macro with any value causes header files to expose
BSD-derived definitions.
In glibc versions up to and including 2.18, defining this macro
also causes BSD definitions to be preferred in some situations
where standards conflict, unless one or more of _SVID_SOURCE,
_POSIX_SOURCE, _POSIX_C_SOURCE, _XOPEN_SOURCE, _XOPEN_SOURCE_EX-
TENDED, or _GNU_SOURCE is defined, in which case BSD definitions
are disfavored. Since glibc 2.19, _BSD_SOURCE no longer causes
BSD definitions to be preferred in case of conflicts.
Since glibc 2.20, this macro is deprecated. It now has the same
effect as defining _DEFAULT_SOURCE, but generates a compile-time
warning (unless _DEFAULT_SOURCE is also defined). Use _DE-
FAULT_SOURCE instead. To allow code that requires _BSD_SOURCE
in glibc 2.19 and earlier and _DEFAULT_SOURCE in glibc 2.20 and
later to compile without warnings, define both _BSD_SOURCE and
_DEFAULT_SOURCE.
_SVID_SOURCE (deprecated since glibc 2.20)
Defining this macro with any value causes header files to expose
System V-derived definitions. (SVID == System V Interface Defi-
nition; see standards(7).)
Since glibc 2.20, this macro is deprecated in the same fashion
as _BSD_SOURCE.
_DEFAULT_SOURCE (since glibc 2.19)
This macro can be defined to ensure that the "default" defini-
tions are provided even when the defaults would otherwise be
disabled, as happens when individual macros are explicitly de-
fined, or the compiler is invoked in one of its "standard" modes
(e.g., cc -std=c99). Defining _DEFAULT_SOURCE without defining
other individual macros or invoking the compiler in one of its
"standard" modes has no effect.
The "default" definitions comprise those required by
POSIX.1-2008 and ISO C99, as well as various definitions origi-
nally derived from BSD and System V. On glibc 2.19 and earlier,
these defaults were approximately equivalent to explicitly
defining the following:
cc -D_BSD_SOURCE -D_SVID_SOURCE -D_POSIX_C_SOURCE=200809
_ATFILE_SOURCE (since glibc 2.4)
Defining this macro with any value causes header files to expose
declarations of a range of functions with the suffix "at"; see
openat(2). Since glibc 2.10, this macro is also implicitly de-
fined if _POSIX_C_SOURCE is defined with a value greater than or
equal to 200809L.
_GNU_SOURCE
Defining this macro (with any value) implicitly defines _AT-
FILE_SOURCE, _LARGEFILE64_SOURCE, _ISOC99_SOURCE,
_XOPEN_SOURCE_EXTENDED, _POSIX_SOURCE, _POSIX_C_SOURCE with the
value 200809L (200112L before glibc 2.10; 199506L before glibc
2.5; 199309L before glibc 2.1) and _XOPEN_SOURCE with the value
700 (600 before glibc 2.10; 500 before glibc 2.2). In addition,
various GNU-specific extensions are also exposed.
Since glibc 2.19, defining _GNU_SOURCE also has the effect of
implicitly defining _DEFAULT_SOURCE. Before glibc 2.20, defin-
ing _GNU_SOURCE also had the effect of implicitly defining
_BSD_SOURCE and _SVID_SOURCE.
_REENTRANT
Historically, on various C libraries it was necessary to define
this macro in all multithreaded code. (Some C libraries may
still require this.) In glibc, this macro also exposed defini-
tions of certain reentrant functions.
However, glibc has been thread-safe by default for many years;
since glibc 2.3, the only effect of defining _REENTRANT has been
to enable one or two of the same declarations that are also en-
abled by defining _POSIX_C_SOURCE with a value of 199606L or
greater.
_REENTRANT is now obsolete. In glibc 2.25 and later, defining
_REENTRANT is equivalent to defining _POSIX_C_SOURCE with the
value 199606L. If a higher POSIX conformance level is selected
by any other means (such as _POSIX_C_SOURCE itself,
_XOPEN_SOURCE, _DEFAULT_SOURCE, or _GNU_SOURCE), then defining
_REENTRANT has no effect.
This macro is automatically defined if one compiles with
cc -pthread.
_THREAD_SAFE
Synonym for the (deprecated) _REENTRANT, provided for compati-
bility with some other implementations.
_FORTIFY_SOURCE (since glibc 2.3.4)
Defining this macro causes some lightweight checks to be per-
formed to detect some buffer overflow errors when employing var-
ious string and memory manipulation functions (for example, mem-
cpy(3), memset(3), stpcpy(3), strcpy(3), strncpy(3), strcat(3),
strncat(3), sprintf(3), snprintf(3), vsprintf(3), vsnprintf(3),
gets(3), and wide character variants thereof). For some func-
tions, argument consistency is checked; for example, a check is
made that open(2) has been supplied with a mode argument when
the specified flags include O_CREAT. Not all problems are de-
tected, just some common cases.
If _FORTIFY_SOURCE is set to 1, with compiler optimization level
1 (gcc -O1) and above, checks that shouldn't change the behavior
of conforming programs are performed. With _FORTIFY_SOURCE set
to 2, some more checking is added, but some conforming programs
might fail.
Some of the checks can be performed at compile time (via macros
logic implemented in header files), and result in compiler warn-
ings; other checks take place at run time, and result in a run-
time error if the check fails.
With _FORTIFY_SOURCE set to 3, additional checking is added to
intercept some function calls used with an argument of variable
size where the compiler can deduce an upper bound for its value.
For example, a program where malloc(3)'s size argument is vari-
able can now be fortified.
Use of this macro requires compiler support, available since gcc
4.0 and clang 2.6. Use of _FORTIFY_SOURCE set to 3 requires gcc
12.0 or later, or clang 9.0 or later, in conjunction with glibc
2.33 or later.
Default definitions, implicit definitions, and combining definitions
If no feature test macros are explicitly defined, then the following
feature test macros are defined by default: _BSD_SOURCE (in glibc 2.19
and earlier), _SVID_SOURCE (in glibc 2.19 and earlier), _DEFAULT_SOURCE
(since glibc 2.19), _POSIX_SOURCE, and _POSIX_C_SOURCE=200809L (200112L
before glibc 2.10; 199506L before glibc 2.4; 199309L before glibc 2.1).
If any of __STRICT_ANSI__, _ISOC99_SOURCE, _ISOC11_SOURCE (since glibc
2.18), _POSIX_SOURCE, _POSIX_C_SOURCE, _XOPEN_SOURCE, _XOPEN_SOURCE_EX-
TENDED (in glibc 2.11 and earlier), _BSD_SOURCE (in glibc 2.19 and ear-
lier), or _SVID_SOURCE (in glibc 2.19 and earlier) is explicitly de-
fined, then _BSD_SOURCE, _SVID_SOURCE, and _DEFAULT_SOURCE are not de-
fined by default.
If _POSIX_SOURCE and _POSIX_C_SOURCE are not explicitly defined, and
either __STRICT_ANSI__ is not defined or _XOPEN_SOURCE is defined with
a value of 500 or more, then
• _POSIX_SOURCE is defined with the value 1; and
• _POSIX_C_SOURCE is defined with one of the following values:
• 2, if _XOPEN_SOURCE is defined with a value less than 500;
• 199506L, if _XOPEN_SOURCE is defined with a value greater than or
equal to 500 and less than 600; or
• (since glibc 2.4) 200112L, if _XOPEN_SOURCE is defined with a
value greater than or equal to 600 and less than 700.
• (Since glibc 2.10) 200809L, if _XOPEN_SOURCE is defined with a
value greater than or equal to 700.
• Older versions of glibc do not know about the values 200112L and
200809L for _POSIX_C_SOURCE, and the setting of this macro will
depend on the glibc version.
• If _XOPEN_SOURCE is undefined, then the setting of
_POSIX_C_SOURCE depends on the glibc version: 199506L, before
glibc 2.4; 200112L, since glibc 2.4 to glibc 2.9; and 200809L,
since glibc 2.10.
Multiple macros can be defined; the results are additive.
STANDARDS
POSIX.1 specifies _POSIX_C_SOURCE, _POSIX_SOURCE, and _XOPEN_SOURCE.
_FILE_OFFSET_BITS is not specified by any standard, but is employed on
some other implementations.
_BSD_SOURCE, _SVID_SOURCE, _DEFAULT_SOURCE, _ATFILE_SOURCE,
_GNU_SOURCE, _FORTIFY_SOURCE, _REENTRANT, and _THREAD_SAFE are specific
to glibc.
HISTORY
_XOPEN_SOURCE_EXTENDED was specified by XPG4v2 (aka SUSv1), but is not
present in SUSv2 and later.
NOTES
<features.h> is a Linux/glibc-specific header file. Other systems have
an analogous file, but typically with a different name. This header
file is automatically included by other header files as required: it is
not necessary to explicitly include it in order to employ feature test
macros.
According to which of the above feature test macros are defined, <fea-
tures.h> internally defines various other macros that are checked by
other glibc header files. These macros have names prefixed by two un-
derscores (e.g., __USE_MISC). Programs should never define these
macros directly: instead, the appropriate feature test macro(s) from
the list above should be employed.
EXAMPLES
The program below can be used to explore how the various feature test
macros are set depending on the glibc version and what feature test
macros are explicitly set. The following shell session, on a system
with glibc 2.10, shows some examples of what we would see:
$ cc ftm.c
$ ./a.out
_POSIX_SOURCE defined
_POSIX_C_SOURCE defined: 200809L
_BSD_SOURCE defined
_SVID_SOURCE defined
_ATFILE_SOURCE defined
$ cc -D_XOPEN_SOURCE=500 ftm.c
$ ./a.out
_POSIX_SOURCE defined
_POSIX_C_SOURCE defined: 199506L
_XOPEN_SOURCE defined: 500
$ cc -D_GNU_SOURCE ftm.c
$ ./a.out
_POSIX_SOURCE defined
_POSIX_C_SOURCE defined: 200809L
_ISOC99_SOURCE defined
_XOPEN_SOURCE defined: 700
_XOPEN_SOURCE_EXTENDED defined
_LARGEFILE64_SOURCE defined
_BSD_SOURCE defined
_SVID_SOURCE defined
_ATFILE_SOURCE defined
_GNU_SOURCE defined
Program source
/* ftm.c */
#include <stdint.h>
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
int
main(int argc, char *argv[])
{
#ifdef _POSIX_SOURCE
printf("_POSIX_SOURCE defined\n");
#endif
#ifdef _POSIX_C_SOURCE
printf("_POSIX_C_SOURCE defined: %jdL\n",
(intmax_t) _POSIX_C_SOURCE);
#endif
#ifdef _ISOC99_SOURCE
printf("_ISOC99_SOURCE defined\n");
#endif
#ifdef _ISOC11_SOURCE
printf("_ISOC11_SOURCE defined\n");
#endif
#ifdef _XOPEN_SOURCE
printf("_XOPEN_SOURCE defined: %d\n", _XOPEN_SOURCE);
#endif
#ifdef _XOPEN_SOURCE_EXTENDED
printf("_XOPEN_SOURCE_EXTENDED defined\n");
#endif
#ifdef _LARGEFILE64_SOURCE
printf("_LARGEFILE64_SOURCE defined\n");
#endif
#ifdef _FILE_OFFSET_BITS
printf("_FILE_OFFSET_BITS defined: %d\n", _FILE_OFFSET_BITS);
#endif
#ifdef _TIME_BITS
printf("_TIME_BITS defined: %d\n", _TIME_BITS);
#endif
#ifdef _BSD_SOURCE
printf("_BSD_SOURCE defined\n");
#endif
#ifdef _SVID_SOURCE
printf("_SVID_SOURCE defined\n");
#endif
#ifdef _DEFAULT_SOURCE
printf("_DEFAULT_SOURCE defined\n");
#endif
#ifdef _ATFILE_SOURCE
printf("_ATFILE_SOURCE defined\n");
#endif
#ifdef _GNU_SOURCE
printf("_GNU_SOURCE defined\n");
#endif
#ifdef _REENTRANT
printf("_REENTRANT defined\n");
#endif
#ifdef _THREAD_SAFE
printf("_THREAD_SAFE defined\n");
#endif
#ifdef _FORTIFY_SOURCE
printf("_FORTIFY_SOURCE defined\n");
#endif
exit(EXIT_SUCCESS);
}
SEE ALSO
libc(7), standards(7), system_data_types(7)
The section "Feature Test Macros" under info libc.
/usr/include/features.h
Linux man-pages 6.7 2023-10-31 feature_test_macros(7)
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